Projection display surface providing speckle reduction

ABSTRACT

A projection display surface for reducing speckle artifacts from a projector having at least one narrow band light source having an incident visible wavelength band, wherein the incident visible wavelength band has an incident peak wavelength and an incident bandwidth, comprising: a substrate having a reflective layer that reflects incident light over at least the incident visible wavelength band; and a fluorescent agent distributed over the reflective layer, wherein the fluorescent agent absorbs a fraction of the light in the incident visible wavelength band and emits light in an emissive visible wavelength band having an emissive peak wavelength and an emissive bandwidth; wherein return light from the projection display surface produced when incident light in the incident visible wavelength band is incident on the projection display surface contains light in both the incident visible wavelength band and emissive visible wavelength band, thereby reducing speckle artifacts.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (96342), entitled: “Projection ApparatusProviding Reduced Speckle Artifacts”, by Barry Silverstein et al., whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to digital image projection andmore particularly to a laser projector with a projection screencontaining fluorescent materials to reduce the speckle noise in thedisplayed image.

BACKGROUND OF THE INVENTION

Laser illumination shows some promise for improving color gamut andachieving needed levels of brightness for digital projection apparatus,including digital projectors capable of providing cinema-quality imagingand pico-projectors offering portable projection for easier sharing ofimages. One recognized problem with projection systems using narrow bandlight sources, however, relates to speckle.

Speckle is a fine scale spatially varying intensity fluctuation that iscaused by random roughness of optical surfaces on the order of awavelength of light. The increased coherence of lasers introduces asignificant effect in projection systems where the roughness createsrandomly phased sub-sources, which interfere together. This randomintensity fluctuation lowers the effective quality of an image,especially at the higher frequencies essentially producing a “shimmereffect” that masks fine details, but also creating an intensitysharpness that is really artificial.

The phenomenon of speckle has been studied in detail by many researchersand a comprehensive summary of knowledge has been published by JosephGoodman in the book “Speckle Phenomena in Optics, Theory and

Application” (Roberts and Company Publishers, Greenwood Village, Colo.,2007). Goodman suggests that full-frame displays should have specklelevels where the standard deviation of the intensity variation is lessthan the magnitude of the least significant bit of the intensityresolution of the modulation device. For Digital Cinema applications 12bit intensity resolutions and contrast ratios of around 2000:1 arecommon. Other cinema standards lean toward different criteria,indicating that speckle “should not be visible”, this can bequantitatively assumed to have the level of speckle to be equivalent tothat of a white light projector on a common screen.

Speckle noise can be quantified in terms of speckle contrast, C, givenin percent as:

$\begin{matrix}{C = {100\left( \frac{I_{std}}{I_{mean}} \right)}} & (1)\end{matrix}$

wherein I_(std) is a standard deviation of intensity fluctuation about amean intensity I_(mean). The speckle contrast for fully developedspeckle is 100%. Speckle reduces the ability of an imaging system toresolve fine spatial detail and causes levels of noise in an image thatcan be highly visually annoying. At worst, without some form ofcorrection, speckle can be sufficiently objectionable to render coherentillumination unsuitable for display purposes.

There have been a number of methods employed for reducing the visibilityof speckle effects in imaging displays. Conventional strategies forspeckle reduction include modifying the spatial or temporal coherence ofthe illumination, superimposing a number of uncorrelated specklepatterns onto each other, or modifying its polarization state. Onemethod provides vibration or oscillatory movement of the display screen.With oscillation above a threshold speed, perceived speckle can besignificantly reduced. Other methods include broadening the spectralline width of the laser illumination and reducing the spatial coherenceby using static and oscillating diffusers or oscillating fibers or byvibrating various optical components in the path of illumination orimaging light.

Goodman has characterized some common approaches to reducing speckle indisplay applications:

-   -   1. Introduce polarization diversity;    -   2. Introduce a moving screen;    -   3. Introduce a specially designed screen that minimizes the        generation of speckle;    -   4. For each color, broaden the spectrum of the sources or use        multiple lasers at slightly different frequencies, thereby        achieving wavelength diversity in the illumination;    -   5. For each color, use multiple independent lasers separated        spatially, thereby achieving angle diversity in the        illumination;    -   6. Overdesign the projection optics as compared with the        resolution of the eye;    -   7. Image a changing diffuser with random phase cells onto the        screen; and    -   8. Image a changing diffuser with deterministic or orthogonal        phase codes onto the screen.

Each of these approaches has some benefits as well as negativeattributes. Some of them apply well for high-end digital cinemaprojection, while others do not. In addition, in many cases a singleapproach may not be effective enough to reduce the speckle belowacceptable thresholds. For example, polarization diversity is notdesirable in many cases, as any projector that requires polarizationeither to modulate the light or to create stereoscopic imaging cannotallow impure states to reach the viewer. Specially designed screens thatenable screen shaking can be effective, however, they requiresignificant modification to the venue that is undesirable. Large screensare especially difficult to modify to enable screen shaking, as theequipment is large and expensive.

Spectrally broadening of the light sources can substantially reduce thelevel of speckle, however, this may be difficult to control in the laserfabrication, as many methods of creating visible solid state sourcesdesired for display applications use frequency double crystals thatcontrol the wavelength to around 1 nm.

Multiple independent lasers can be a very good approach, but depends onthe number of elements used to control the speckle. This does not workwell over the entire range from low-light-level to high-light-levelprojection system, as a 1000 lumen projector needs to be as speckle freeas a 10,000 lumen projector, yet the number of sources may be 10 timesas high. For example, Mooradian et al, disclose improved speckleperformance when using Novalux Extended Cavity Surface Emitting Lasers(NECSELS), in the article “High power extended vertical cavity surfaceemitting diode lasers and arrays and their applications” (Micro-OpticsConference, Tokyo, Japan, 2005). In this case 30 to 40 independent(incoherent to each other) emitters reduced the speckle down to severalpercent. While the speckle is reduced with larger number of emitters itis not always reduced to white light levels required by the stringentdigital cinema requirements.

In U.S. Pat. No. 7,296,897, Mooradian et al., entitled “Projectiondisplay apparatus, system, and method,” discloses individual andcombined techniques to reduce laser speckle similar to those describedby Goodman. First increasing the number of lasers that are substantiallyincoherent with respect to each other. Second, spectral broadening ofthe lasers may be used. (This technique is also described in U.S. Pat.No. 6,975,294 to Manni et al.) Third, individual lasers in an array maybe designed to operate with multiple frequencies, phase, and directional(angular) distributions. Finally an optical element may be used toscramble the direction, phase and polarization information. As describedearlier, increasing the number of lasers is effective at reducingspeckle, however the effect is incomplete. The additional methodsdescribed are generally difficult to implement, expensive or undesirableoptically.

U.S. Pat. No. 7,244,028 to Govorkov et al., entitled “Laser illuminatedprojection displays,” describes the use at least one laser delivered toa scanning means that increases the laser beam divergence temporallyinto a lens that delivers the light to a beam homogenizer thatilluminates a spatial light modulator. This reduces the laser speckle toacceptable levels when combined with a screen that has at least onefeature to further reduce speckle. Temporally varying the laser beamdivergence is generally a good means of reducing speckle, however it toorequires the modification of the screen for complete speckle reduction.This is undesirable for general projection purposes.

U.S. Pat. No. 7,116,017 to Ji et al., entitled “Device for reducingdeterioration of image quality in display using laser,” describes aspecific device consisting of a vibrating mirror in the light pathbetween the laser and the screen. This alone will not reduce speckle toacceptable levels. Commonly assigned U.S. Pat. No. 6,445,487 to Roddy etal., entitled “Speckle suppressed laser projection system using amulti-wavelength Doppler shifted beam,” describes methods that usefrequency modulation of the lasers in conjunction with a device todeviate the beam angularly in time. This method requires lasermodulation that may not be practical or possible for all laser sources.Similarly the application focuses on using an acousto-optic modulatorfor angular deviation. These devices are very expensive and can onlyhandle certain laser types and sizes.

Numerous methods for reducing speckle have been described in the priorart. U.S. Pat. No. 6,747,781 to Trisnadi et al., entitled “Method,apparatus, and diffuser for reducing laser speckle,” discloses moving adiffusing element that is positioned at an intermediate image plane thatsubdivides image pixels into smaller cells having different temporalphase. Commonly-assigned U.S. Pat. No. 6,577,429 entitled “Laserprojection display system” to Kurtz et al. discloses using anelectronically controllable despeckling modulator to providecontrollable, locally randomized phase changes with a linear SLM. U.S.Pat. No. 6,323,984 entitled “Method and apparatus for reducing laserspeckle” to Trisnadi et al. discloses speckle reduction using awavefront modulator in the image plane. U.S. Pat. No. 5,313,479 entitled“Speckle-free display system using coherent light” to Florence disclosesillumination of a light valve through a rotating diffuser. U.S. Pat. No.4,256,363 to Briones, entitled “Speckle suppression of holographicmicroscopy,” and U.S. Pat. No. 4,143,943 to Rawson, entitled “Rearprojection screen system,” each disclose apparatus that reduce speckleby moving diffusive components that are within the projection path.Commonly-assigned U.S. Patent Application Publication 2009/0284713 toSilverstein, et al., entitled “Uniform speckle reduced laser projectionusing spatial and temporal mixing,” teaches using a temporally varyingoptical phase shifting device in the optical path to reduce speckle in adigital cinema system.

While conventional methods for speckle reduction may have someapplicability to laser-based projection systems, there are drawbacks tothese approaches that constrain image quality and reduce overallcontrast as well as adding cost and complexity to projection apparatus.Any type of modification to components in the imaging path, for example,can require significant redesign, can complicate component packaging,and risks the introduction of noise or vibration into optical and signalpaths of projector components.

The problem of speckle reduction is further complicated becausedifferent types of spatial light modulators (SLMs) are being used fordigital projection. Three types of SLMs are used in practice:point-scan, line-scan and frame-by-frame. Point-scan projectors displayan image by raster scanning a single pixel at a time. A number ofprojectors use grating light valves (GLVs) or grating electromechanicalsystems (GEMS) that generate images using diffractive gratings that havetiny mechanical members that are variably actuated in order to form animage. The image from such a device is scanned onto the display surface,a single line at a time. These modulators are advantaged with respect tosimplicity and cost, and therefore are desirable for use in consumerdevices such as pico-projectors. However, they present problems due tothe energy density that can be delivered which limits the amount oflight that can safely be projected. Other projectors employ reflectiveor transmissive liquid-crystal devices (LCDs). These SLMs project acomplete image frame at a time. Still other projection apparatus usedigital micromirror devices with two-dimensional arrays ofmicro-electromechanical reflectors, such as the Digital Light Processor(DLP) from Texas Instruments, Inc., Dallas, Tex. DLP devices similarlyform a complete image frame at a time. These area-type devices areadvantaged in delivering less energy density to the screen offeringsafer operation. Because images are formed in different ways using thesedifferent SLMs and projection technologies, solutions that compensatefor speckle with one type of SLM may not be as effective when used in aprojector that uses a different type of SLM for forming images.

A number of different approaches have been developed which use speciallydesigned screens to reduce speckle. U.S. Pat. No. 6,122,023 to Chen etal., entitled “Non-speckle liquid crystal projection display,” disclosesa projection screen which includes a liquid crystalline material. Whendriven with an AC voltage the liquid crystalline materials vibrateslightly which causes the speckle pattern to change quickly which causesthe observed speckle noise by the viewer to be reduced.

U.S. Pat. No. 7,304,795 to Yavid and Stem, entitled “Image Projectionwith Reduced Speckle Noise,” discloses a projection screen whichincludes a plurality of optical resonator cavities which trap incidentlaser light for a time greater than the coherence time and forgenerating a time varying interference pattern in which speckle noise isreduced.

U.S. Pat. No. 5,473,469 to Magocs and Baker, entitled “Front projectionscreen with lenticular front surface,” discloses a front projectionscreen for use with a laser projector which includes a lenticular lensarray on the front surface of the screen which incorporates lightscattering particles to form a diffusion region and a reflector on itsback surface. Since incident light rays traverse different portions ofthe diffusion region in different directions which increases thelikelihood that the ray will incorporate a scattering particle, specklenoise is reduced.

The use of projection screens incorporating color changing materials isdescribed in the following art. U.S. Pat. No. 7,414,621 to Yavid et al.,entitled “Method and Apparatus for Controllably Producing a LaserDisplay,” discloses a raster scanned laser display for projecting animage on a screen incorporating at least one phosphor at the screen forreflecting light with a wavelength different from the wavelength of theincident laser beam which emits light in the ultra-violet or IRwavelength region of the spectrum. Complete absorption of the laser beamis required by the phosphor in order to fully utilize this approach.

U.S. Pat. No. 6,987,610 to Piehl, entitled “Projection Screen,”discloses a projection screen comprising a substrate having thereon oneor more fluorescent materials that emit visible light with an incidenceof one or more ranges of visible light and absorb visible light in atleast one other range of wavelengths that is not included in the one ormore ranges and one or more absorption materials disposed between thesubstrate and the one or more fluorescent materials that reflectwavelengths of light in the one or more ranges and absorb wavelengths oflight that are not included in the at least one other range nor in theone or more ranges.

U.S. Patent Application Publication 2008/0172197 to Skipor et al.,entitled “Single laser multi-color projection display with quantum dotscreen,” discloses a display comprising a passive screen printed with apattern of different color quantum dots that is excited by rasterscanning a single UV laser beam over the screen.

U.S. Pat. No. 7,474,286 to Hajjar et al., entitled “Laser Displays usingUV-Excitable Phosphors Emitting Visible Colored Light,” discloses adisplay system using at least one scanning laser beam to excite one ormore fluorescent materials on a screen in the form of parallel phosphorstripes which emit light to form images. An alignment verificationsensor is also required to verify that the laser light modulation timingis correctly aligned with the phosphor stripes during raster scanning ofthe laser over the screen surface. In a related disclosure, U.S. PatentApplication Publication 2008/0291140 to Kent et al., entitled “DisplaySystems Having Screens with Optical Fluorescent Materials,” furtherteaches that the fluorescent materials may include phosphor materials orquantum dots.

U.S. Patent Application Publication 2008/0048936 to Powell et al.,entitled “Display and display screen configured for wavelengthconversion,” discloses a display screen including an array of coupletscontaining a wavelength converting material. The couplets are configuredto receive light at a first wavelength and responsively emit light at asecond wavelength preferentially in a direction.

U.S. Patent Application Publication 2009/0262308 to Ogawa, entitled“Light source unit and projector,” discloses a projector, which includesfirst and second light sources comprising light emitting diodes or asolid-state light emitting devices for emitting light in each of twopredetermined wavelength bands and a third light source formed by aphosphor which transmits light of the first light source and absorbslight emitted from the second light source. In this case there is nophosphor material on the screen.

Thus, it can be appreciated that speckle presents a recurring problemthat must be addressed in projection apparatus design when laserillumination is used. Conventional speckle compensation approaches addcost and complexity to projector design, and generally reduce imagequality with respect to projector output. There is, then, a need for aspeckle compensation mechanism that can be used for a broad range ofimaging technologies and that does not impact projector design.

SUMMARY OF THE INVENTION

It is an object of the present invention to advance the art of digitalimage projection. With this object in mind, an embodiment of the presentinvention is characterized by a projection display surface for reducingspeckle artifacts from a projector having at least one narrow band lightsource having an incident visible wavelength band, wherein the incidentvisible wavelength band has an incident peak wavelength and an incidentbandwidth, comprising:

a) a substrate having a reflective layer that reflects incident lightover at least the incident visible wavelength band; and

b) a fluorescent agent distributed over the reflective layer, whereinthe fluorescent agent absorbs a fraction of the light in the incidentvisible wavelength band and emits light in an emissive visiblewavelength band having an emissive peak wavelength and an emissivebandwidth; wherein the emissive bandwidth is wider than the incidentbandwidth and is at least five nanometers in width;

wherein return light from the projection display surface produced whenincident light in the incident visible wavelength band is incident onthe projection display surface contains light in both the incidentvisible wavelength band and emissive visible wavelength band, therebyreducing speckle artifacts by the mechanism of spectral broadening.

The apparatus of the present invention has the advantage that it isindependent of the image-forming technology that is used within theprojector. It is equally well-suited for use with projection systemsthat use spatial light modulators that scan a linear image onto thedisplay surface as well as for projection systems that form a completetwo-dimensional image at a time.

It has the additional advantage that it does not add any cost orcomplexity to the projection apparatus itself.

It is a further advantage of the present invention that it reducesspeckle with little perceptible impact on image quality.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a digital projection apparatususing the projection display surface of the present invention;

FIG. 2 is a graph showing an example emission spectrum for a singlecolor channel in the digital projection apparatus;

FIG. 3 is a graph showing an idealized example of a fluorescent emissivewavelength band produced by absorbing light in an incident wavelengthband;

FIG. 4 is a graph showing an idealized return light spectrum resultingfrom a small Stokes shift;

FIG. 5 is a diagram showing enlarged front and side views of a displaysurface conditioned with a sparsely distributed fluorescing agent;

FIG. 6A shows an image of an example speckle pattern produced with nofluorescent agent;

FIG. 6B shows an image having reduced speckle produced according to thepresent invention;

FIG. 7 is a graph plotting measured speckle contrast and mean code valuefor green laser light as a function of optical density for a rhodamine6G fluorescent agent;

FIG. 8 is a graph plotting measured speckle contrast and mean code valuefor red, green and blue laser light as a function of optical density fora rhodamine 6G fluorescent agent;

FIG. 9 is a graph of the measured spectral radiance versus wavelengthfor green laser light incident on an uncoated screen sample;

FIG. 10 is a graph of the measured spectral radiance versus wavelengthfor green laser light incident on a screen sample with an OD=0.04coating of a rhodamine 6G fluorescent agent;

FIG. 11 is a graph of the measured spectral radiance versus wavelengthfor green laser light incident on a screen sample with an OD=0.30coating of a rhodamine 6G fluorescent agent;

FIG. 12 is a graph of the measured spectral radiance versus wavelengthfor red, green and blue laser light incident on an uncoated screensample;

FIG. 13 is a graph of the measured spectral radiance versus wavelengthfor red, green and blue laser light incident on a screen sample with anOD=0.04 coating of a rhodamine 6G fluorescent agent;

FIG. 14 is a graph of the measured spectral radiance versus wavelengthfor red, green and blue laser light incident on a screen sample with anOD=0.30 coating of a rhodamine 6G fluorescent agent;

FIG. 15 shows a graph of the CIE 1931 2° color matching functions; and

FIG. 16 shows a flow chart for color calibrating a digital projectionsystem according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the detailed information that follows, it is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art. Figures shown and described hereinare provided to illustrate key principles of operation and componentrelationships along their respective optical paths according toembodiments of the present invention and may not show actual size orscale. Some exaggeration may be necessary in order to emphasize basicstructural relationships or principles of operation.

Embodiments of the present invention address the problem of specklereduction by adapting the response of a display screen or other type ofdisplay surface to incident narrow band light from a projector. Byredistributing a portion of the incident light energy to emissivematerials on the display screen surface, embodiments of the presentinvention effectively expand the spectral bandwidth of the displayedimage, thereby reducing speckle artifacts with little or no perceptibleimpact on brightness and color quality.

The simplified schematic of FIG. 1 shows one type of a projectionapparatus 10 with a projection lens 20 that projects a light beam 22 toform an image on a display surface 30, having a reflective layer 26provided on a substrate 25. In some embodiments, the substrate 25 ismade of a reflective material. In this case, the reflective layer 26 canbe the top surface of the substrate. Viewers 150, who are positionedunderneath light beam 22, can view the projected image on displaysurface 30. Depending on their distance from the screen and otherfactors, they may also perceive speckle artifacts and other imageartifacts such as metameric failure artifacts.

The projection apparatus 10 has three or more color channels, typicallyred (R), green (G) and blue (B). Each color channel has a narrow bandlight source, 16 r, 16 g and 16 b, and a corresponding spatial lightmodulator 12 r, 12 g and 12 b, respectively. In a preferred embodiment,the spatial light modulator 12 r, 12 g, and 12 b are digitalmicro-mirror devices, such as the well known Digital Light Processor(DLP) spatial light modulators available from Texas Instruments, Inc. ofDallas, Tex.

Modulated light from each of three or more color channels is combinedonto the same optical path, optical axis O, using a combining elementsuch as a dichroic combiner 14. This same basic model also applies foran LCD or other type of spatial light modulator used with such a system,with a different orientation of the LCD modulator relative to itscoherent light source.

Each light source 16 r, 16 g and 16 b is a narrow-band light source(such as a laser light source or an LED light source) having a visiblewavelength band characterized by a peak wavelength and a bandwidthproviding some amount of energy over a small range of nearbywavelengths. The graph of FIG. 2 illustrates a wavelength band 40corresponding to the emission spectrum of a representative laser used asan illumination source in a digital projector, such as that shown inFIG. 1. In this example, the laser is shown as a Green laser, havingpeak wavelength λ1 of 532 nm, but having energy at wavelengths that areslightly to each side of this central value. The width of the wavelengthband 40 is characterized by a bandwidth Δλ1 (e.g., the full-width halfmaximum bandwidth). This same basic relationship also applies for Redand Blue lasers, over their corresponding wavelength bands. Typicallaser bandwidths used in laser projectors are in the range of 0.05-0.50nm.

This high concentration of light over a very narrow wavelength band hasadvantages for providing a broad color gamut. However, because the laserlight is highly coherent, this same narrow-band characteristic is also afactor that contributes to the formation of perceptible speckle in thedisplayed image. Speckle can be reduced by increasing the spread oflight energy over the spectrum. The approach of the present inventionattempts to broaden this energy distribution in a controllable manner,without unduly compromising image quality.

In the present invention, this broadening of the energy distribution isachieved by conditioning the response of the display screen (displaysurface 30 in the system of FIG. 1) to incident light. This conditioningof the screen can be achieved in a number of ways. In one embodiment, afluorescent agent 27 such as a fluorescent dye is distributed over thereflective layer 26 of the display surface 30 provides this spectralbroadening function, supplementing the reflective properties of thereflective layer 26.

Fluorescent dyes are materials that absorb light energy at a firstwavelength and, in response to this absorbed energy, emit light energyat a second wavelength that is spectrally shifted from the firstwavelength. Fluorescent dyes are useful, for example, for locating andtracing various biochemical substances in molecular imagingapplications. These applications use the fluorescent dye response knownas Stokes shift. Stokes shift relates to the difference in wavelength influorescence response between the transmitted and partially absorbedfirst light energy at λ1 and the emitted second light energy λ2.

For biomedical imaging applications, highly selective filters are usedin order to separate incident light wavelengths of the excitation lightfrom fluorescent light wavelengths. To help further facilitate thisseparation, researchers use materials that fluoresce at wavelengths thatare markedly higher than the excitation wavelength. For the purposes ofthe present invention, however, only a slight shift in wavelength due tothe Stokes shift is desirable.

Referring to FIG. 3, the phenomenon of low Stokes shift for one type offluorescent dye is shown. Incident visible light in an incidentwavelength band 42 having an incident peak wavelength λ1 and an incidentbandwidth Δλ1 is absorbed by the fluorescent dye. The fluorescent dyethen emits light in an emissive wavelength band 44 having an emissivepeak wavelength λ2 and an emissive bandwidth λ2. The emissive peakwavelength λ2 is shifted relative to the incident peak wavelength λ1 bya Stokes shift Δλs. In accordance with the present invention, both theincident wavelength band 42 and the emissive wavelength band 44 willgenerally be in the visible wavelength range, which extends fromapproximately 400-700 nm.

In a low Stokes-shift condition, such as that represented in FIG. 3, theamount of the Stokes shift Δλs between λ1 and λ2 can be as low as a fewnm. In the context of the present invention, for broadening the energydistribution of perceived light in order to reduce speckle, a shift ofless than about 40 nm works well. A more preferable Stokes shift wouldbe at a smaller value, such as a shift of less than about 20 nm or lessthan about 10 nm. This amount of color shift that results can beimperceptible to the viewer of the projected image. Further, since theemissive wavelength band 44 typically has an extended tail, keeping thespectral shift smaller reduces undesirable residual light that mayoverlap into a neighboring color spectrum. But, because of the slightfrequency difference between the original laser light and the emittedlight that is excited, speckle artifacts in the on-screen image arereduced.

Preferably, the emissive bandwidth Δλ2 is wider than the incidentbandwidth Δλ1 and is at least 5 nm in width and no more than about 50 nmin width. The wider bandwidth is helpful to provide speckle reduction byspectral broadening.

Exemplary dyes usable for the present invention include Alexa Fluor®Dyes from Life Technologies Corporation, Carlsbad, Calif. For the Greencolor channel, for example, Alexa Fluor 532 dye has an absorption peakwavelength λ1 at 532 nm and an emission peak wavelength λ2 at about 555nm.

The amount of the fluorescent agent 27 distributed over the reflectivelayer 26 can be adjusted to control the amount of light in the incidentwavelength band 42 that is absorbed and produces emitted light in theemissive wavelength band. Generally, it will be desirable to use arelatively low amount of the fluorescent agent 27 such that only afraction of the incident light is absorbed such that the return lightfrom the display surface 30 contains both light in the incidentwavelength band 42 that is reflected from the reflective layer 26 andlight in the emissive wavelength band 44 that is emitted by thefluorescent agent 27.

While the previously mentioned references by Piehl (U.S. Pat. No.6,987,610), Skipor et al. (U.S. Patent Application Publication2008/0172197), Hajjar et al. (U.S. Pat. No. 7,474,286), Kent et al.(U.S. Patent Application Publication 2008/0291140) and Powell et al.(U.S. Patent Application Publication 2008/0048936) each teach the use ofdisplay screens including fluorescent agents, that there are severalimportant differences between the present invention and the prior artconfigurations. In each of these references, the purpose of thefluorescent agent is to change the color of the incident light into anew color. In some cases, invisible ultraviolet radiation is convertedto visible light. In contrast, the method of the present inventionprovides return light that is shifted by only a small interval relativeto the incident light. Preferably, the magnitude of the shift is smallenough such that the incident light and the return light would beperceived by a human observer to have the same color name (e.g., “red,”“green” or “blue”).

Furthermore, in the prior art references the fluorescent agents aredesigned to absorb substantially all of the incident laser light so thatthe return light contains only emissive light. In contrast, the returnlight in the present invention contains light in both the incidentwavelength band 42 and the emissive wavelength band 44. This feature isuseful to produce spectral broadening in order to reduce the visibilityof speckle artifacts. None of the prior art projection screens teachconfigurations that provide speckle reduction.

The example of FIG. 3 illustrates applying the method of the presentinvention to provide speckle reduction for a green color channel havinga green narrow-band light source. In some configurations, significantreductions in the visibility of speckle artifacts can be achieved byusing a single fluorescent agent to provide spectral broadening for onlya single color channel (i.e., the green color channel or some othercolor channel). For example, speckle artifacts are often more visible inone color channel more than the other color channels. Therefore,reducing the speckle artifacts in that color channel can providesignificant image quality improvements even if no speckle reduction isprovided for the other color channels.

In other configurations, the same speckle reduction principle can beapplied for a plurality of color channels by distributing multiplefluorescent agents over the reflective layer. For example, a redfluorescent agent that absorbs light in the red incident wavelength bandand emits light in a corresponding red emissive wavelength band can bedistributed over the reflective layer together with the greenfluorescent agent. Alternately, multiple fluorescing agents 27 can bedistributed over the display surface 30 to operate on a given colorchannel. For example, two fluorescent agents 27, one with a smallpositive Stokes shift, and the other with a small negative (anti-Stokes)shift could provide spectral broadening on either side of the incidentwavelength band 42.

The characteristic response shown in FIG. 3 is idealized and representsthe power of the excited fluorescent energy at a level approximatelyequal to that of the initial excitation beam. In practice, such abalanced relationship is not needed; what is useful is obtainingsufficient fluorescence to slightly broaden the spectral distribution ofthe color light so that speckle artifacts are eliminated, or at leastreduced to imperceptible levels.

It is not necessary that the return light have two distinct peakwavelengths as was shown in the example of FIG. 3. For example, FIG. 4shows an idealized example where an incident wavelength band 42stimulates an emissive wavelength band 44 have a relatively small Stokesshift. Additionally, the power of the return light in the emissivewavelength band is smaller than the power of the return light in theincident wavelength band 42. In this case, a resulting return lightwavelength band 46 has the same peak wavelength λ1 as the incidentwavelength band 42, but has a broadened spectrum with a broadenedbandwidth of Δλ_(B). An advantage of this arrangement is that the colorshift of the return light will be much smaller than that for the exampleof FIG. 3.

The fluorescent agent (e.g., the fluorescent dye) can be applied to thesurface of a projection screen in a number of ways. For example, afluorescent dye can be dispersed or suspended within an applied coating.A number of practical observations apply for the use of a fluorescentdye in this manner:

-   (i) The dye molecules will generally be much smaller (nm scale) than    the pixel size (mm scale) on the display screen. Therefore,    dispersion of the fluorescent dye molecules as particulates or in a    suspension enables the application of the fluorescent dye onto the    display screen surface at suitable concentrations without affecting    the image resolution.-   (ii) In one embodiment, a separate fluorescent dye is applied to the    projection screen surface for each of the color channels (e.g., red    green, and blue). The different fluorescent dyes can be combined and    applied as part of the same coating. Alternately they can be applied    in separate coatings. In alternate embodiments, a fluorescent dye is    applied for a single color channel only (e.g., green) and no    conditioning of the projection screen is provided for the other    color channels (e.g., blue and red). It can be appreciated that    various combinations of conditioning for this purpose can be used,    including treatment for any one, any two, or all three color    channels. For four-color projection systems, even more permutations    are possible.-   (iii) Relatively low amounts of fluorescent dye are typically    required in order to provide the needed effect. For example, a dye    concentration sufficient to absorb about 10% of the light in the    incident wavelength band has been found to provide substantial    speckle reduction in a typical configuration. Generally, the    concentration of the fluorescent dye should be chosen to absorb    between 2% and 40% of the light in the incident wavelength band. The    optimum concentration of the fluorescent dyes for each color channel    will be a function of a number of factors including the speckle    visibility of the color channel and the degree of spectral    broadening provided by the Stokes shift for a particular fluorescent    dye.-   (iv) Where the fluorescent dye is applied onto an existing screen    formulation, some type of added coating protection may be useful to    provide durability. In one embodiment, an overcoat layer (e.g., a    polymer overcoat) is provided over the top of the fluorescent agent    27 (FIG. 1) to provide protection for the fluorescent agent.    Preferably, the material that is used for the overcoat layer should    not exhibit perceptible birefringence. In alternate embodiments, the    fluorescent agent 27 can be suspended within the overcoat layer. The    overcoat layer is generally used to protect the fluorescent dye from    environmental concerns. In some cases, the overcoat layer may also    be doped or coated with an ultraviolet light blocker to provide    further protection for the fluorescent dyes.

In general, fluorescent agents emit light that is at a lower energy(i.e., at a longer wavelength) than the excitation energy. Thisrelationship is shown in FIG. 3, for example. There are also fluorescingmaterials for which the emitted energy is at a lower wavelength. Thistype of response has been termed “anti-Stokes” shift. Materials thatexhibit this type of response could alternately be used for embodimentsof the present invention.

Examples of fluorescent dyes that are suitable for use with projectedlight in a red color channel include those given in Table 1. Generally,a fluorescent dye should be selected where the peak absorptionwavelength is closely matched to the emissive peak wavelength for thenarrow-band light source. Similarly, examples of fluorescent dyes thatare suitable for use with a green color channel include those given inTable 2, and examples of fluorescent dyes that are suitable for use witha blue color channel include those given in Table 3.

TABLE 1 Example fluorescent dyes for use with a red color channel. PeakAbsorption Peak Emission Fluorescent Dye Wavelength (nm) Wavelength (nm)BODIPY 630/650 625 640 BODIPY 650/665 646 660 Alexa Fluor 633 632 647Alexa Fluor 635 633 647 Alexa Fluor 647 650 668

TABLE 2 Example fluorescent dyes for use with a green color channel.Peak Peak Absorption Emission Wavelength Wavelength Fluorescent Dye (nm)(nm) Alexa Fluor 500 503 525 Alexa Fluor 514 518 540 Alexa Fluor 532 531554 BODIPY R6G 528 550 BODIPY 530/550 534 554 BODIPY TMR 542 5744′,5′-Dichloro-2′,7′-dimethoxy-fluorescein 522 5502′,7′-Dichloro-fluorescein 510 532 Eosin 524 544 Erythrosin 530 555Oregon Green 514 511 530 Rhodamine 6G 525 555 Rhodamine Green dye 502527 2′,4′,5′,7′-Tetrabromosulfonefluorescein 528 5442′,4,7,7′-Tetrachlorofluorescein, 521 536 succinimidyl ester (TET) EYFP(enhanced yellow fluorescent 512 529 protein)

TABLE 3 Example fluorescent dyes for use with a blue color channel. PeakAbsorption Peak Emission Fluorescent Dye Wavelength (nm) Wavelength (nm)Dialkylaminocoumarin 435 475 Sytox blue 445 475 Pacific Blue dye 410 455Alexa Fluor 405 402 421 Cascade Blue 400 420

FIG. 5 is a diagram showing enlarged front and side views of a displaysurface 30 conditioned with a fluorescent agent 27 according to analternate embodiment. In this case, the fluorescent agent is distributedover a reflective layer 26 on a substrate 25 in a patterned fashion ofsparsely and randomly distributed fluorescent dots 24. Alternatively,the fluorescent dots 24 could also be arranged in a more uniformlyordered matrix. The reflective layer 26 reflects the incident light asin a conventional film projection surface. An optional protectivecoating 28 is also shown encapsulating the fluorescent dots 24. In thecontext of the present invention, a sparse distribution of fluorescentdots is an arrangement in which individual fluorescent dots 24 do notinteract with each other. This implies that the fluorescent dots 24cover no more than about 40% of the display surface 30. (Similarly, whenthe fluorescent agent 27 is applied in a uniform coating, a sparsecoating of the fluorescent agent 27 will be defined to be one wherethere is no aggregation of fluorescent agent molecules.)

The number and density of the fluorescent dots 24 can be selected toachieve the desired level of absorption of the light in the incidentwavelength band (generally between 2% and 40%). For example, if thefluorescent dots 24 were sufficiently dense so as to absorb virtuallyall of the light in the incident wavelength band, and if it were desiredto absorb 9% of the incident light, then the number of fluorescent dots24 should be chosen to cover 9% of the surface area of the displaysurface 30.

For configurations where multiple fluorescent agents are used to providespectral broadening for a plurality of color channels, each fluorescentdot 24 can have a single fluorescent agent. Alternately, eachfluorescent dot 24 can include a combination of the fluorescent agents.Generally, it will be desirable that the size of the fluorescent dots 24are small relative to the size of a projected pixel so that every pixelwill produce return light having both reflected incident light andemissive light. The fluorescent dots 24 are shown to be round dots inthis example, but they could be formed using any convenient shape.

The size and number of fluorescent dots 24, as well as the concentrationof the fluorescent agent in the fluorescent dots 24, can be adjusted tocontrol the relative amounts of reflected incident light and emissivelight in the return light. In one embodiment of the FIG. 5configuration, the fluorescent agent 27 is applied in a highconcentration such that the majority of the light in the incidentwavelength band that falls on the fluorescent dots 24 is absorbed andused to stimulate emissive light in the emissive wavelength band.

To test the speckle reduction characteristics for projection screensformed according to the method of the present invention, sample screenswere formed based on a Hurley Screen MW-16 screen material. HurleyScreen MW-16 material is a heavy gauge titanium dioxide pigmented vinylfilm. It has a smooth surface with microscopic embossing for maximumlight distribution and has a gain of 1.0 with a viewing angle of 50degrees.

Rhodamine 6G was coated onto the Hurley Screen MW-16 screen material asfollows. Rhodamine 6G dye was dissolved in 1-butanol and diluted to makecoatings with a variety of optical densities. Square samples of HurleyScreen MW-16 were cut to dimensions of 2″ by 2″ and spin coated at 2000RPM with the diluted rhodamine 6G dye solutions. Samples were thenheated to 50° C. and dried for 30 minutes before testing. Samples wereprepared having optical densities of 0.0, 0.04, 0.12, 0.30, 0.60 forincident light having a peak wavelength of 532 nm.

The five different screen samples were mounted onto a poster board andall samples were illuminated simultaneously under uniform illuminationconditions using a laser projection system. The laser projection systemused green lasers having a wavelength of 532 nm, blue lasers having awavelength of 465 nm and red lasers having a wavelength of 637 nm.

Speckle contrast measurements for the five different screen samples wereperformed simultaneously using a Canon-EOS Rebel XSi 12.2 Mpixel camera(4272×2848 pixels) with a 28-135 mm zoom lens set to a focal length of135 mm. The camera was mounted on a tripod located 10 ft from the screenduring the measurements. Raw camera images were used throughout andconverted to 16 bit TIFF files for analysis.

Each pixel in the captured digital image files corresponded to a screenarea of 175×175 microns. For comparison, for normal 20/20 vision theeye's angular resolution is approximately 1.0 arc minutes, and someindividuals have better visual acuity down to 0.3 arc minutes. For a 1.0arc minute angular resolution and a 10 ft viewing distance, the normaleye resolution would be about 880 microns, and the eye resolution forthe best visual acuity would be about 260 microns. Therefore, it can beseen that the captured digital image contains sufficient spatial detailto adequately characterize the visibility of the speckle patterns.

Speckle contrast was calculated by locating each of the five screenpatches mounted on the poster board in the captured digital cameraimage. Calculations were performed using 200×200 camera pixels regionsselected from within each screen patch. Mean and standard deviation ofthe raw linear camera code values in all three color channels weremeasured to calculate the speckle contrast in each of the wavelengthbands. Since we only have a green emitter in these samples only thegreen color channel is active to provide speckle reduction by themechanism of spectral broadening. This is demonstrated in Table 4 whichshows data for white light, green light, red and blue incident light.

TABLE 4 Measured speckle contrast for rhodamine 6G coated screensMeasured Speckle Contrast vs OD of rhodamine 6G dye Exposure Laser OD ODOD OD OD Ratio OD Condition Time (sec) F/# Color(s) 0.00 0.04 0.12 0.300.60 0.04/0.00 1 1/30 16 G 0.0885 0.0750 0.0749 0.0996 0.1314 0.8469 21/20 16 G 0.0794 0.0655 0.0652 0.0887 0.1287 0.8251 3 1/60 16 G 0.05230.0424 0.0422 0.0535 0.0696 0.8101 4 1/60 11 G 0.0521 0.0435 0.04390.0518 0.0654 0.8339 5 1/30 11 RGB 0.0194 0.0157 0.0158 0.0204 0.03260.8085 6 1/30 16 RGB 0.0319 0.0269 0.0275 0.0387 0.0579 0.8445 7 1/30 22RGB 0.0414 0.0355 0.0363 0.0512 0.0808 0.8576 8 1/30 22 RGB 0.04110.0351 0.0372 0.0500 0.0798 0.8537 9 1/30 22 RGB 0.0414 0.0350 0.03650.0521 0.0790 0.8448 10 1/30 22 R 0.0214 0.0213 0.0209 0.0206 0.02010.9977 11 1/30 11 B 0.0207 0.0206 0.0227 0.0260 0.0272 0.9978

The first column in Table 4 shows the measurement condition number. Themeasurement condition includes the camera exposure time in seconds shownin column 2, the camera F/# shown in the column 3, and the color of theincident laser(s) used to illuminate the screen is shown in column 4.Columns 5-9 of Table 4 show measured speckle contrast calculated fromEq. (1) for the uncoated screen sample (OD 0), and the coated screensamples having rhodamine 6G dye layers with optical densities of 0.04,0.12, 0.30 and 0.60, respectively. For conditions 1-4, which utilizegreen laser light only, and conditions 5-9, which utilize combined red,green and blue laser light, the speckle contrast data is reported usingonly the green color channel data. For condition #10, which utilizes redlaser light only, the speckle contrast data is reported using the redcolor channel data; and for condition #11, which utilizes blue laserlight only, the speckle contrast data is reported using the blue colorchannel data. The last column of Table 4 shows the ratio of the OD 0.04data to the OD 0.00 data.

This data shows that the best speckle reduction generally occurred forthe OD 0.04 samples, where there was a 14-20% speckle reduction over theuncoated samples when green laser light is incident on the screensamples. While the numbers would appear to suggest that this improvementis fairly small on a percentage basis, the visual impression of theimprovement is actually much more significant than these numbers reveal.While the visible speckle levels with the green laser light are quiteobjectionable for the OD 0.00 sample, the visible speckle has beendramatically reduced for the OD 0.04. This disparity between themagnitude of the speckle contrast and the visibility of the speckleartifacts to a human observer is presumably a reflection of deficienciesin the speckle contrast metric, which does not take into account thefrequency content of the speckle or the frequency response of the humanvision system.

From Table 4, it can also been seen that the speckle levels forconditions #10 and #11 remain essentially constant. This is consistentwith the fact that the rhodamine 6G fluorescent agent is only effectivefor absorbing the green laser light. Therefore, there will be nospectral broadening for the red and blue color channels, and nocorresponding reduction in the speckle artifacts.

Table 5 shows the measured mean camera code values of the various coatedscreen samples corresponding to the data shown in Table 4. Columns 1-4in Table 5 are the same as in Table 4. Columns 5-9 of Table 5 show themean camera code values for the screen samples with rhodamine 6G opticaldensities of 0.00, 0.04, 0.12, 0.30, and 0.60, respectively,corresponding to the speckle contrast measurements in Table 4. (Thedigital camera saturates at 65535 counts. During the measurements themaximum camera counts per color channel were also measured. The exposuretimes and F/# combinations shown in Tables 4 and 5 were selected so thatno pixels with saturated code values were observed for any of thereported data.) It can be seen that for higher dye densities, there isan observable reduction in the camera code values, reflecting a lowerscreen brightness level due to the fact that the fluorescent dye isabsorbing more light than it is emitting.

TABLE 5 Measured mean code values for rhodamine 6G coated screensMeasured mean code value vs OD of rhodamine 6G dye Exposure Laser OD ODOD OD OD Condition Time (sec) F/# Color(s) 0.00 0.04 0.12 0.30 0.60 11/30 16 G 30352 29652 26503 19325 14091 2 1/20 16 G 41351 40754 3743228151 21004 3 1/60 16 G 26629 26455 22750 14482 8040 4 1/60 11 G 4209741994 37586 25340 14654 5 1/30 11 RGB 55816 54610 50719 42032 30280 61/30 16 RGB 39251 38389 33806 25234 14759 7 1/30 22 RGB 24645 2398220122 13560 6664 8 1/30 22 RGB 24898 24267 20369 13738 6846 9 1/30 22RGB 24969 24356 20504 13894 6877 10 1/30 22 R 30104 31028 30868 3111029430 11 1/30 11 B 39522 39900 38055 34388 24967

FIGS. 6A and 6B shows images of the green color plane of the 200×200pixel regions of the Hurley Screen MW-16 material samples without (dyeoptical density=0.00) and with a low optical density coating (dyeoptical density=0.04) of rhodamine 6G fluorescent dye respectively. Bothof these images were obtained using illumination and camera set upcondition 2 from Table 4, with green only incident laser light of 532nm. While it is uncertain whether the differences will be clear in theprinted figures, there is a dramatic decrease in the visibility of thespeckle artifacts for FIG. 6B relative to FIG. 6A in the original imagesused to produce these figures.

FIG. 7 shows a graph of the measured speckle contrast 52 as a functionof screen optical density of rhodamine 6G for illumination and cameraset up condition #2. Note that the speckle contrast improves for lowoptical density coatings. At OD of 0.3 and above the speckle contrastwas found to be worse than that for the screen without any rhodamine 6Gcoating. While the amount of spectral broadening for these samples willbe larger, and therefore the amount of speckle reduction due to spectralbroadening should be better, there are apparently other sources ofspeckle artifacts that arise at high dye concentrations which begin todominate the reduced speckle levels that would be expected due to thespectral broadening.

It is likely the increased speckle levels arise from non-uniformities inthe thickness or density of the fluorescent dye coating. At higheroptical densities there is a tendency for fluorescent dye molecules toaggregate which can result in fluorescence quenching. Rhodamine 6G isknown to form dimers, trimers and higher aggregates in high opticaldensity solutions which result in fluorescence quenching. (For moreinformation see the article “The fluorescence quenching mechanisms ofRhodamine 6G in concentrated ethanolic solution” by F. López Arbeloa etal. in Journal of Photochemistry and Photobiology A: Chemistry, Vol. 45,Pages 313-323, 1988). Therefore, it would be expected that improveddeposition or coating methods for providing the fluorescent agents 27 tothe display surface 30 could allow higher dye densities to be usedwithout observing the upswing in the speckle contrast. An example ofsuch an improved coating method is described in the article “FluorescentPlasma Nanocomposite Thin Films Containing Nonaggregated Rhodamine 6GLaser Dye Molecules” by A. Barranco and P. Groening, Langmuir, Vol. 22,pp 6719-6722 (2006).

Also shown in FIG. 7 is a curve plotting the measured mean code values54 in the green color channel as a function of screen optical density ofrhodamine 6G for illumination. It is observed that the amount of greenlight reflected from the screen is monotonically decreasing as theoptical density of the rhodamine 6G coating increases.

FIG. 8 shows a similar graph of the measured speckle contrast 62 and themeasured mean code values 64 in the green color channel as a function ofscreen optical density of rhodamine 6G for camera set up condition #6.While condition #2 utilizes only incident green laser light, condition#6 utilizes white incident light formed by activating the red, green andblue laser sources. The white light condition of FIG. 8 exhibits thesame trends for speckle reduction and mean camera counts as the greenlight only condition of FIG. 7. Note that since the speckle contrastcalculated for this graph used only the green color channel of thedigital camera image, therefore it will not provide an indication of thevisibility of any speckle which may be present in the red and blue colorchannels.

For condition #10, which used only red laser light illumination, onlythe red color channel is relevant for the calculation of specklecontrast. In this case, the speckle contrast and mean code values inTables 4 and 5 were determined using only the red color channel data.Likewise, for condition #11 which used only blue laser lightillumination, only the blue color channel is relevant for thecalculation of the speckle contrast and the mean code values. In bothcases, it can be seen that the speckle in the red and blue colorchannels was relatively unaffected by the rhodamine 6G fluorescent agentas would be expected.

Color measurements were obtained with a Photo Research PR650 SpectralColorimeter. The spectral radiance in W/(sr·m²) was measured for greenlaser light, as well “white laser light” made by a combination of red,green and blue laser light. FIG. 9 shows a graph of the measuredspectral radiance versus wavelength for 532 nm green laser lightincident on an uncoated screen sample. Since there is only one laserturned on in the projector there is only a single green reflection peak72 at 532 nm in the spectral radiance plot. Even though the laserbandwidth is less than 0.5 nm the measured spectrum appears muchbroader, which is due to the spectral bandwidth of the instrument, whichis about 10 nm.

FIG. 10 shows a graph of the spectral radiance versus wavelength for the532 nm laser light incident on the rhodamine 6G coated screen samplehaving an optical density of OD=0.04. As in FIG. 9, a green reflectionpeak 82 can be observed corresponding to the 532 nm laser light.However, it can be seen that the magnitude of the spectral radiance atthe green reflection peak 82 has decreased slightly from that of theuncoated film measurement, and that some of the light has been shiftedto longer wavelengths as indicated by the appearance of a greenfluorescence band 84. As will be discussed below, the shift of thereturn light from the green reflection peak 82 into the greenfluorescence band 84 results in a shift in x and y chromaticitycoordinates.

FIG. 11 shows a graph of the spectral radiance versus wavelength for the532 nm laser light incident on the rhodamine 6G coated screen samplehaving an optical density of OD=0.3. As in FIG. 10, both a greenreflection peak 92 and a green fluorescence band 94 can be observed. Forthis higher optical density coating, the peak intensity of the greenreflection peak 92 has decreased much further and the green fluorescenceband 94 centered at 560 nm is much more pronounced.

FIG. 12 shows a graph of the spectral radiance versus wavelength forwhite light screen illumination formed by combining light from a 465 nmblue laser light source, a 532 nm green laser light source and a 637 nmred laser light source incident on an uncoated screen sample. Threepeaks are now observed in the spectral radiance data: a blue reflectionpeak 102 at 465 nm, a green reflection peak 104 at 532 nm and a redreflection peak 106 at 637 nm. (Even though the bandwidths of each ofthe lasers are less than 0.5 nm they all appear to be broader due to thespectral bandwidth of the instrument.)

FIG. 13 shows a graph of the spectral radiance versus wavelength for thecase where the same white light screen illumination as in FIG. 12 isincident on the screen sample coated with rhodamine 6G an opticaldensity of OD=0.04. As in FIG. 12, a blue reflection peak 112, a greenreflection peak 114 and a red reflection peak 116 can be observed,although the magnitude of the spectral radiance at the green reflectionpeak 114 has decreased slightly from that of the uncoated screen shownin FIG. 12. Additionally, it can be seen that some of this light hasbeen shifted to longer wavelengths between 532 and 575 nm in greenfluorescence band 128. The magnitude of the blue reflection peak 112 andthe red reflection peak 116 are essentially unchanged relative to theuncoated screen measurement shown in FIG. 12.

Preferably, the peak wavelength of the green fluorescence band 128 iscloser to the green reflection peak 114 than to either the bluereflection peak 112 or the red reflection peak 116. To maintainseparation between the different primaries, it is generally desirablethat the green fluorescence band 128 extend to no less than about 30 nmaway from the peak wavelengths of the blue reflection peak 112 and thered reflection peak 116. The fluorescent agent that produces the greenfluorescence band 118 should generally absorb a fraction of the greenincident light, but should not absorb any appreciable amount of the redand blue incident light (e.g., less than 10%).

Similarly, FIG. 14 shows a graph of the spectral radiance versuswavelength for the case where the same white light screen illuminationas in FIG. 12 is incident on the screen sample coated with rhodamine 6Gto an optical density of OD=0.12. As in FIG. 13, the spectral radianceincludes a blue reflection peak 122, a green reflection peak 124, a redreflection peak 126 and a green fluorescence band 128. Again, themagnitude of the blue reflection peak 122 and the red reflection peak126 are the same as for the uncoated screen shown in FIG. 12. Thedecrease in the magnitude of the green reflection peak 124 shown in FIG.14 is even greater than that shown in FIG. 13. The magnitude of thegreen fluorescence band 128 is also greater in FIG. 14 than in FIG. 13.These observations confirm that the green laser light is mostlyresponsible for the fluorescence from the screen coated with rhodamine6G dye.

The PR650 Spectral Colorimeter also measures the luminance and the CIEchromaticity coordinates x and y. The chromaticity and luminance arecalculated based on the 1931 CIE 2° color matching functions, x(λ),y(λ), and z(λ), by the following procedure. FIG. 15 is a graph showingthe x(λ) color matching function 136, the y(λ) color matching function134, and the z(λ) color matching function 132 plotted as a function ofwavelength. First the CIE XYZ tristimulus values are calculated as:

$\begin{matrix}{{X = {\sum\limits_{\lambda}{{S(\lambda)}{\overset{\_}{x}(\lambda)}\Delta \; \lambda}}}{Y = {\sum\limits_{\lambda}{{S(\lambda)}{\overset{\_}{y}(\lambda)}\Delta \; \lambda}}}{Z = {\sum\limits_{\lambda}{{S(\lambda)}{\overset{\_}{z}(\lambda)}\Delta \; \lambda}}}} & (2)\end{matrix}$

where S(λ) is the spectral power distribution and Δλ is the wavelengthinterval between the wavelength samples. The CIE 1931 color matchingfunctions are usually tabulated in 1 nm increments, and in this case thewavelength interval would be 1 nm. The measured spectral radiancedistributions shown in FIGS. 9-14 are equivalent to the spectral powerdistribution. The Y term in Eq. (2) is equivalent to the luminance ofthe measured color on the screen and is typically reported in footlamberts (fl) or candelas per meter² (cd/m²). The chromaticitycoordinates x, y, z are calculated by the relationships:

$\begin{matrix}{{x = \frac{X}{X + Y + Z}}{y = \frac{Y}{X + Y + Z}}{z = {\frac{Z}{X + Y + Z} = {1 - x - y}}}} & (3)\end{matrix}$

Tables 6-9 show measured colorimetry data (luminance Y and chromaticityvalues x, y) for various illumination conditions on the uncoated screensample and a selection of the rhodamine 6G coated film samples. Table 6shows colorimetry data for the case of illuminating the screen sampleswith 532 nm green laser light. At an optical density of OD=0.04, it canbe seen that there is a small color shift and a small luminancedecrease. As the optical density of the dye increases, there areincreasingly larger color shifts and decreases in screen luminance onthe screen. Thus, increasing the optical density decreases the opticalefficiency, which is undesirable.

TABLE 6 Measured colorimetry for green laser light illumination onRhodamine 6G dye coated screens. OD Y (fl) x y 0.00 456.0 0.1799 0.7830.04 443.7 0.2173 0.7504 0.12 402.8 0.276 0.6994 0.30 287.2 0.3356 0.643

Table 7 shows colorimetry data for the case of illuminating the screensamples with “white” laser light made up of combined blue (465 nm),green (532 nm) and red (637 nm) laser light. At an optical density ofOD=0.04, the color shift and luminance decrease is small. However, asthe optical density of the dye increases, it can be seen there areincreasingly larger color shifts and decreases in overall screenluminance.

TABLE 7 Measured colorimetry for “white” laser light illumination onRhodamine 6G dye coated screens. OD Y (fl) x y 0.00 655.0 0.2996 0.31770.04 648.8 0.3125 0.3094 0.12 602.9 0.3352 0.2999

Table 8 shows colorimetry data for the case of illuminating the screensamples with 637 nm red laser light. It was found that as the opticaldensity of the rhodamine 6G dye was increased, there was no significanteffect on the luminance and color of the red laser illumination. Thesame observations hold true for Table 9, which shows colorimetry datafor the case of illuminating the screen samples with 465 nm blue laserlight.

TABLE 8 Measured colorimetry for red laser light illumination onRhodamine 6G dye coated screens. OD Y (fl) x y 0.00 159.5 0.7089 0.28570.04 158.8 0.7089 0.2858 0.12 157.9 0.7089 0.2859

TABLE 9 Measured colorimetry for blue laser light illumination onRhodamine 6G dye coated screens. OD Y (fl) x y 0.00 48.1 0.1378 0.04410.04 47.4 0.1397 0.0482 0.12 45.7 0.1449 0.0572

The data for the rhodamine 6G dye coated onto the Hurley MW-16 screenmaterial show that at low optical densities between 0.04 and 0.12 thereis a 15-20% reduction in speckle contrast. In some embodiments, the dyeconcentration corresponding to the lowest speckle visibility can bedetermined by measuring a series of samples to determine theconfiguration that minimizes the speckle contrast (or some other measureof speckle visibility). In other embodiments, it may be desirable toconsider other factors, such as the amount of color shift and luminancedegradation, when determining the best dye concentration. At an opticaldensity of 0.12, there is a noticeable decrease in the measuredluminance reflecting off of the screen. There is also a greater colorshift relative to the 0.04 optical density case. Therefore, in someapplications it will be preferable to select an optical density at thelower end of this range to provide the optimal performance.

The selection of appropriate fluorescent dyes for a particularapplication should take into account the performance attributes thathave been discussed above (speckle contrast reduction, screen luminancedecrease, color shift (Stokes shift Δλs) and emissive bandwidth Δλ2), aswell as other factors such as cost, availability, dye fadecharacteristics, physical durability and toxicity.

Even at relatively low dye concentrations, measurable color shifts canbe observed as can be seen from Tables 6 and 7. To obtain the optimalimage quality, it is desirable to compensate for any such color shiftsintroduced by the use of the fluorescent agents to reduce the speckleartifacts. This can be accomplished by using a color processor to applyan appropriate color correction transform to account for the spectralcharacteristics of the return light, which includes the reflectedincident light together with the light emitted by the fluorescent agent.

When a projector is set up, it usually has encoded into it a normalizedprimary matrix, which is based on the spectral distributions of theprojector light sources. In order to correct for any color shiftsinduced by the viewing conditions, which would include the color shiftsinduced by the screen, it is necessary to calibrate the projector andscreen under normal viewing conditions together as a system. FIG. 16shows a color calibration process flow chart 200 describing steps thatcan be used to calibrate the projector and screen under normal viewingconditions according to a preferred embodiment. It will be recognized byone skilled in the art that many other variations of this colorcalibration process can be used in accordance with the presentinvention.

First, a measure spectral response of primaries step 202 is performed.This involves measuring the spectral response of each of the individualprimary light sources with a spectrum measuring instrument such as aspectrophotometer or a spectroradiometer. In order to correct for anycolor shifts induced by the viewing conditions, which include the colorshifts induced by the screen, it is necessary to measure each of theindividual primary light sources in the context of the expected viewingenvironment by measuring return light from the viewing screen ratherthan directly measuring the spectrum of the light sources themselves.For the present example, it will be assumed that the laser projector hasred, green and blue primaries, although this method can be generalizedto apply to laser projectors with other sets of primaries. The measuredspectral response for the i^(th) primary light sources will be given byS_(i)(λ), where i=R, G, B for the red, green and blue primary lightsources, respectively.

Next, the primary tristimulus values and chromaticity coordinates aredetermined using a calculate colorimetry of primaries step 204. Thetristimulus values X_(i), Y_(i), and Z_(i) for the i^(th) primary lightsource are calculated follows:

$\begin{matrix}{{X_{i} = {\sum\limits_{\lambda}{{S_{i}(\lambda)}{\overset{\_}{x}(\lambda)}\Delta \; \lambda}}}{Y_{i} = {\sum\limits_{\lambda}{{S_{i}(\lambda)}{\overset{\_}{y}(\lambda)}\Delta \; \lambda}}}{Z_{i} = {\sum\limits_{\lambda}{{S_{i}(\lambda)}{\overset{\_}{z}(\lambda)}\Delta \; \lambda}}}} & (4)\end{matrix}$

where these equations are adapted from Eq. (2). The chromaticitycoordinates for each of the primary light sources x_(i), y_(i), z_(i)are calculated from the relationships:

$\begin{matrix}{{x_{i} = \frac{X_{i}}{X_{i} + Y_{i} + Z_{i}}}{y_{i} = \frac{Y_{i}}{X_{i} + Y_{i} + Z_{i}}}{z_{i} = {1 - x_{i} - y_{i}}}} & (5)\end{matrix}$

A primary matrix P is then determined using a determine primary matrixstep 206. This step forms the primary matrix P from the chromaticitycoordinates of the primary light sources as follows:

$\begin{matrix}{P = {\begin{bmatrix}x_{R} & x_{G} & x_{B} \\y_{R} & y_{G} & y_{B} \\z_{R} & z_{G} & z_{B}\end{bmatrix}.}} & (6)\end{matrix}$

A determine normalized primary matrix 208 is then used to determine anappropriately normalized primary matrix. For this step, it will beassumed that the x and y chromaticity coordinates for the desired aimwhite point for the projector system are given as x_(w), y_(w), z_(w). Awhite reference vector W_(ref) is defined as:

$\begin{matrix}{W_{ref} = \begin{bmatrix}{x_{W}/y_{W}} \\1 \\{z_{W}/y_{W}}\end{bmatrix}} & (7)\end{matrix}$

As an example the DCI reference white point chromaticity values arex_(W)=0.314, y_(W)=0.351 and z_(W)=0.335. Now a color coefficientdiagonal matrix C is calculated using the relationship:

C=I ₃·(P ⁻¹ ·W _(ref))   (8)

where I₃ is a 3×3 identity matrix and P⁻¹ is the inverse matrix ofprimary matrix P. The normalized primary matrix NPM is then given by:

NPM=P·C   (9)

A store normalized primary matrix step 210 is then used to store thenormalized primary matrix NPM into memory for use in color correctingdisplayed images. In some embodiments, the normalized primary matrix NPMis encoded into the projector's firmware. The normalized primary matrixNPM can be combined with a matrix associated with the color encoding ofthe input image in order to determine a primary conversion matrix thatcan be used to convert linear input RGB values into linear RGB valuesappropriate for display on the projector/display screen system. In someembodiments, the combined primary conversion matrix may be stored inmemory rather than the normalized primary matrix NPM.

Next, a determine primary power adjustment factors step 212 is used tocalculate power adjustment factors for each primary light source thatare used to achieve the desired white point and luminance level. Thepower adjustment factors can be calculated as follows. For the whitepoint, the tristimulus value is equivalent to the sum of the tristimulusvalues for the individual primary light sources. The tristimulus valuesdetermined in the calculate colorimetry of primaries step 204 (X_(R),Y_(R), Z_(R) for the red primary light source, X_(G), Y_(G), Z_(G) forthe green primary light source and X_(B), Y_(B), Z_(B) for the blueprimary light source) were measured at an initial set of source powerlevels P_(R0), P_(G0), P_(B0), where P_(R0) is the red primary lightsource's initial power level, P_(G0) is the green primary light source'sinitial power level, and P_(B0) is the blue primary light source'sinitial power level. Generally, the power levels are measured in Watts,although other units of measurement can also be used in accordance withthe present invention. As the relative powers between the threeprimaries are varied the color of the white point will be changed.

Coefficients A_(R), A_(G) and A_(B) are defined as the ratios of a newpower level to that of their initial power levels P_(R0), P_(G0) andP_(B0). When the power levels are adjusted the new white pointtristimulus values X_(n), Y_(n) and Z_(n) are calculated by therelationship:

X _(n) =A _(R) X _(R) +A _(G) X _(G) +A _(B) X _(B)

Y _(n) =A _(R) Y _(R) +A _(G) Y _(G) +A _(B) Y _(B)

Z _(n) =A _(R) Z _(R) +A _(G) Z _(G) +A _(B) Z _(B)   (10)

The new white point chromaticity values x_(n) and y_(n) can becalculated by:

$\begin{matrix}{{x_{n} = \frac{{A_{R}X_{R}} + {A_{G}X_{G}} + {A_{B}X_{B}}}{\begin{matrix}{{A_{R}\left( {X_{R} + Y_{R} + Z_{R}} \right)} +} \\{{A_{G}\left( {X_{G} + Y_{G} + Z_{G}} \right)} + {A_{B}\left( {X_{B} + Y_{B} + Z_{B}} \right)}}\end{matrix}}}{y_{n} = \frac{{A_{R}Y_{R}} + {A_{G}Y_{G}} + {A_{B}Y_{B}}}{\begin{matrix}{{A_{R}\left( {X_{R} + Y_{R} + Z_{R}} \right)} +} \\{{A_{G}\left( {X_{G} + Y_{G} + Z_{G}} \right)} + {A_{B}\left( {X_{B} + Y_{B} + Z_{B}} \right)}}\end{matrix}}}} & (11)\end{matrix}$

Next, the three power ratio coefficients A_(R), A_(G) and A_(B) can bedetermined by solving for the values that satisfy the conditions:

x_(n)=x_(w)

y_(n)=y_(w)

Y_(n)=Y_(d)   (12)

where Y_(d) is the desired luminance level measured from the screen. Thepower ratio coefficients A_(R), A_(G) and A_(B) can be determined usingany method for solving systems of equations known in the art. Forexample, nonlinear solvers in software packages such as Matlab orMathematica can be used to solve for the desired values.

Once the power ratio coefficients A_(R), A_(G) and A_(B) are determined,an adjust primary power levels step 214 is used to determine new powerlevels for each of the primary light sources that will produce thespecified white point chromaticity and luminance level. The new powerlevels for the primaries are calculated by:

P_(R)=A_(R)P_(R0)

P_(G)=A_(G)P_(G0)

P_(B)=A_(B)P_(B0)   (13)

where P_(R), P_(G) and P_(B) are the new power levels for the red, greenand blue primaries.

The exemplary color calibration process of FIG. 16 is equally applicableto variations of the present invention where more than one fluorescentagent 27 is used on the display surface 30. For example, it can be usedfor the case where a different fluorescent agent 27 is used for eachcolor channel. It can also be used for the case where both Stokes andanti-Stokes fluorescing agents are used in at least one color channel.

In addition to fluorescent dyes and related compounds, other types ofmaterials can also be applied to the display surface 30 (FIG. 1) as thefluorescent agent 27 for providing the equivalent low Stokes shifteffect. For example, quantum dots can be used as the fluorescent agent27. Quantum dots are semiconductor nanocrystals whose fluorescentresponse characteristics, including spectral shift, are a factor ofcrystal size. Commonly, quantum dots materials are fabricated usingsmall particles of inorganic semiconductors having particle sizes lessthan about 40 nm. More information about quantum dots can be found inU.S. Patent Application Publication 2008/0217602 to Kahen, entitled“Quantum dot light emitting device.”

Exemplary quantum dot materials include, but are not limited to, smallparticles of CdS, CdSe, ZnSe, InAs, GaAs and GaN. Similar to thefluorescent dyes described with reference to FIG. 3, the quantum dot,when excited by light radiation at first wavelength λ1 emits afluorescent response radiation at second wavelength λ2. Unlikefluorescent dyes, however, the emitted wavelengths depend on the quantumdot particle size, the particle surface properties, and the inorganicsemiconductor material that is used. Advantageously, because of theirsmall size, quantum dot materials dispersed in host materials exhibitlow optical backscattering.

Colloidal dispersions of highly luminescent core/shell quantum dots havebeen fabricated by a number of researchers over the past decades (forexample, see B. O. Dabbousi et al, “(CdSe)ZnS Core-Shell Quantum Dots:Synthesis and characterization of a size series of highly luminescentnanocrystallites,” J. Phys. Chem. B 1997, Vol. 101, 9463-9475, 1997). Alight emitting core can be composed of type IV (Si), III-V (InAs), orII-VI (CdTe) semiconductive material. For emission in the visible partof the spectrum, CdSe is a preferred core material since by varying thediameter of the CdSe core in the range 1.9 to 6.7 nm, the emissionwavelength can be tuned from 465 to 640 nm.

As is well known in the art, visible emitting quantum dots can befabricated from other material systems, such as, doped ZnS. The lightemitting cores are made by chemical methods well known in the art.Typical synthetic routes for this fabrication include decomposition ofmolecular precursors at high temperatures in coordinating solvents,solvothermal methods, and arrested precipitation. The semiconductorshell surrounding the core is typically composed of type II-VIsemiconductive material, such as, CdS or ZnSe. The shell semiconductoris typically chosen to be nearly lattice-matched to the core materialand to have valence and conduction band levels such that the core holesand electrons are largely confined to the core region of the quantumdot. Preferred shell material for CdSe cores is ZnSe_(x)S_(1-x), with xvarying from 0.0 to ˜0.5. Formation of the semiconductor shellsurrounding the light emitting core is typically accomplished via thedecomposition of molecular precursors at high temperatures incoordinating solvents or using reverse micelle techniques.

For colloidal synthesis of semiconductor nanocrystals such as quantumdots, a three-component system of precursors, organic surfactants, andsolvents is used. After a reaction medium is heated to a sufficientlyhigh temperature, the precursors chemically transform into monomers.Once the monomers reach a high enough supersaturation level, nanocrystalgrowth begins with a nucleation process. Temperature during this growthprocess is one factor in determining optimal conditions for nanocrystalgrowth and must be high enough to allow for rearrangement and annealingof atoms during the synthesis process while low enough to promotecrystal growth. Another important factor that is stringently controlledduring nanocrystal growth is the monomer concentration.

The growth process of nanocrystals can occur in two different regimes,“focusing” and “defocusing”. At high monomer concentrations, thecritical size (the size where nanocrystals neither grow nor shrink) isrelatively small, resulting in growth of nearly all particles. In thisregime, smaller particles grow faster than large ones (since largercrystals need more atoms to grow than small crystals) resulting in“focusing” of the size distribution to yield nearly monodisperseparticles. The size focusing is optimal when the monomer concentrationis maintained such that the average nanocrystal size present is alwaysslightly larger than the critical size. When the monomer concentrationis depleted during growth, the critical size becomes larger than theaverage size and the distribution “defocuses”.

There are colloidal methods suitable for many different semiconductors,including cadmium selenide, cadmium sulfide, indium arsenide, and indiumphosphide. These quantum dots can contain as few as 100 to 100,000 atomswithin the quantum dot volume, with a diameter of 10 to 50 atoms. Thiscorresponds to a diameter of about 2 to 10 nm. Larger quantum dots canalso be formed, with dimensions in excess of 100 nm.

As has been noted, the spectral response of the quantum dot can beengineered by controlling the geometrical size of the nanocrystalstructure. Further control is available by altering quantum dot shapeand changing other properties that relate to its quantum confinementpotential. As a fluorescing agent in embodiments of the presentinvention, a particular quantum dot material can be formed anddimensioned so that it fluoresces at a favorable wavelength relative tothe incident color light, as was shown for fluorescent dyes in theexample of FIGS. 3 and 4. In one embodiment, the same semiconductormaterial is used in quantum dot form for multiple color channels, withthe quantum dots for each color channel formulated and sizedappropriately. In an alternate embodiment, quantum dots for differentcolor channels are from different semiconductor materials.

As mentioned earlier, speckle reduction is important to enable highimage quality imaging in both digital cinema projection and in consumerprojection. In digital cinema applications, the level of acceptablespeckle is probably lower than that of the consumer space. In thisviewing environment, the room is dark which, in general, opens up theviewer's pupil, lessening the generated speckle to the eye. However, theimage quality must be comparatively high so as not to distract from thestory. In the consumer space, however, the ambient light levels arelikely to be significantly higher, such as in an office environment, orworse outdoor daylight. In this case the viewer's pupil is significantlysmaller, increasing the visibility of the laser speckle. While thetolerance of laser speckle under these viewing circumstances areprobably higher than that of a movie theatre, the conditions thatgenerate speckle are much worse. One way to assist in improving theconsumer viewing experience would be to combine a screen with ambientlight rejection along with speckle reduction. Ambient light rejectionmay be done many ways. One way is to create a surface structure thatoptically redirect light from directions other than direct on-axisprojection away from viewer positions. This can be done by positioningphysical lenses or holographic optical elements o on top of the screen.

An alternate embodiment would be to incorporate further dyes or pigmentsin addition to the fluorescent agents 27. The laser spectrum, as notedearlier, is narrow and it is also desirable to maintain a relativelynarrow band of the spectrally broadened speckle reduced light.Therefore, there are significant wavelength bands between the originalincident laser illumination wavelength bands and the emissive wavelengthbands that are unused by the projection system. Yet ambient light whichis relatively broadband is reflected by a conventional screen material.Light absorbing agents, such as dyes or pigments, selected to absorblight in unused visible wavelength bands would significantly benefit theusability of these screens. Preferably, the light absorbing pigmentsshould be selected so that their absorption bands do not substantiallyoverlap with any of the incident laser illumination wavelength bands orany of the emissive visible wavelength bands corresponding to thefluorescent agents 27. The contrast ratio would be substantiallyenhanced by some elimination of this ambient light. Additionally,speckle visibility would be further reduced by the increase of pupilsize accorded with lower ambient light. This increased pupil size has adirect correlation to reduced speckle visibility. The light absorbingagents are preferably distributed over the reflective layer 26 in thesame way as the fluorescent agents 27. In one embodiment, the lightabsorbing agents are included in the same coating layer as thefluorescent agents. In another embodiment light absorbing dots aredistributed over the reflective layer together with fluorescent dots 24.

Embodiments that use the method and apparatus of the present inventionhelp to compensate for speckle by conditioning the projection displaysurface with a fluorescent agent having a low Stokes shift that ispreferably within about 25 nm of the peak laser frequency.Advantageously, the projection surface of the present invention can beused with any of a number of types of digital projection apparatus thatuse laser illumination in one or more color channels. The projectionsurface 30 can also be advantageously used for speckle reduction inother types of projection systems having narrow bandwidth light sources,including LEDs or visible wavelength super luminescent diodes.

As another aspect and advantage of the present invention, it isrecognized that the display surface 30 having sparsely deposited, smallStokes shift, fluorescing agents 27 can also reduce the effects ofmetameric failure artifacts associated with differences in observerspectral sensitivity. In the field of color science, metamerism is thevisual perception of color matching for color stimuli having differentspectral power distributions. Said another way, colors are said to bemetamers if they appear identical to the human eye even though they havevery different spectra. Color imaging system rely on the phenomenon ofmetamerism to produce color images having the desired color appearancebecause the reproduced color spectrum will generally not match the colorspectrum of an original scene. However, the amounts of the colorantsused by the color imaging system can be adjusted to produce a colorwhich will appear to match the original scene color.

The visual phenomenon of metamerism depends on the interaction of thelight source spectra with the optical properties of the materials thelight reflects from, and the color perception of the observer's eyes.Metameric failure artifacts occur when different observers perceivedifferent colors for the same color stimuli. There are variousclassifications for metameric failure artifacts, including illuminantmetameric failure artifacts (when two material samples have match colorappearance when viewed under one light source but not another) andobserver metameric failure artifacts (when observer color visiondifferences cause reporting of different colors or hues).

Laser projection displays, like that of FIG. 1, bring various potentialadvantages to the field of image projection, including, importantly, agreatly expanded color gamut as compared to other technologies. However,when the color channel spectra are narrowed using lasers (FIG. 12), andthe color gamut is expanded, the degree of observer metameric failureartifacts can become more pronounced such that different observers mayperceive the displayed colors with significant differences.

Observer metameric failure artifacts can be better understood byconsidering the 1931 CIE 2° standard observer color matching functions,shown in FIG. 15. These curves depict a representative color perceptionexpectation for the human population, which was derived empirically bycolor-matching experiments with a small population of observers. Thecolor matching functions (CMFs) of FIG. 15 represent an “average” or“standard” color-normal human observer known as the CIE 1931 standardobserver. Although subsequent studies have addressed concerns anddeficiencies in the 1931 results, and improved CMFs have been published,the 1931 CMFs of FIG. 15 are still pertinent and are widely used.

However, individual human observers each have their own unique CMFs,which lead to observer to observer differences in color perception. Forexample, the article “Minimizing observer metamerism in displaysystems”, by Rajeev Ramanath (Color Research & Application, Vol. 34, pp.391-398, 2009) provides a figure with comparative CMFs for a sampling ofmodeled observers and individual observers for which data is availablefrom various published studies. The figure shows significant localvariations (as much as 5-10%) in color responsivity as a function ofwavelength. Ramanath explores the comparative susceptibility to observermetameric failure artifacts when color content is viewed with differentelectronic display devices, including CRT displays, LCD, DLP and LEDbased displays, a CCFL (cold cathode fluorescent lamp) based display,and a laser display. Ramanath concludes that observer metameric failureartifacts can occur more frequently as the display spectrum narrows(smaller FWHM) or the number of modes in the display spectrum increases.As a result, the laser display and CCFL display, which have narrow ormulti-modal spectra, have the greatest propensity to observer metamericfailure artifacts. By comparison, the CRT display and the LED display,which have ˜73 nm and ˜28 nm bandwidths respectively, exhibit low andmedium potentials for observer metameric failure artifacts respectively.

In consideration of Ramanath, it can be seen that the display screen 30of the present invention, having sparsely deposited, small Stokes shift,fluorescent agents 27 will have the benefit of reducing the degree ofobserver metameric failure artifacts in addition to reducing speckleartifacts in image content displayed on the screen by a laser projector10 due to the broadened spectra in the color channels.

Considering again the exemplary fluorescence-broadened green spectra ofFIGS. 10 and 11, it is seen that the green fluorescence bands 84 and 94can broaden the spectra by 40 nm or more. The FIG. 11 green fluorescenceband 94, which has a higher relative energy, would be more valuable inreducing the degree of observer metameric failure artifacts observed byviewers 150 of projector 10, as compared to the exemplary fluorescencebroadened green spectra of FIG. 10. In a general sense, the broader andstronger (higher intensity) that the fluorescent band becomes; the morethe observer metameric failure artifacts will be reduced.

Expanding on this, the exemplary data of FIG. 7 suggests that to obtainthe spectral broadening of FIG. 11, which was produced by a screen withan OD 0.3 fluorescent agent coating, that the spectral broadening toreduce observer metameric failure artifacts may exceed that which isdesirable for reducing speckle perceptibility. Additionally, thespectral broadening provided by the fluorescent agent 27 will have theeffect of reducing the color gamut of the projection system. Therefore,it will generally be necessary to select an optimum fluorescent agentconcentration that balances the improvement in the observer metamericfailure artifacts with the improvement in the speckle and the reductionin the color gamut. Preferably, a balanced approach is desired, wherefluorescence broadening at the screen using fluorescent agents 27contributes to speckle reduction and observer metameric failure artifactreduction, but without incurring significantly reductions of theprojector color gamut.

The results presented in FIGS. 7-14 indicate that a range offluorescence broadened spectra can be produced depending on the coatingparameters. However, as previously noted, these results representinitial experiments using a particular screen material (Hurley MW-16), aparticular dye (Rhodamine 6G), and a non-optimized spin coating process.Other combinations of screen materials, fluorescent agents, and coatingor patterning processes will yield different results. The selection ofthe fluorescent agent 27, relative to it Stokes shift Δλs andfluorescent bandwidth Δλ2 will significantly determine the impact oncolor gamut, speckle reduction, and observer metameric failure artifactreduction. However, these spectral properties can also be effected bythe organizational structure imparted to the fluorescent agent 27 by thecoating process and the screen surface structure. Additionally, it isnoted that coating two or more fluorescing agents 27 for a given colorchannel (e.g., one with a small positive Stokes shift, and one with asmall negative (anti-Stokes) shift) can provide spectral broadening oneither side of the primary spectrum, which can expand the spectralbroadening to enhance both speckle reduction and observer metamericfailure artifact reduction without necessarily changing theconcentration or fill factor of the fluorescing agents 27 on the displaysurface 30. It will be obvious to one of ordinary skill in the art thatall of these factors can be co-optimized to balance the impact on colorgamut, speckle reduction, and observer metameric failure artifactreduction, as well as to address other system design criteria.

For some applications, the light beam 22 projected by the projectionapparatus 10 onto the display surface 30 uses polarized light. Forexample, many stereoscopic projection systems alternately project lightbeams in two different orthogonal polarization states. The viewer 150wears glasses with polarizing filters so that the light of onepolarization state is viewed by one eye and the light of the otherpolarization state is viewed by the other eye. In such cases, it isimportant that the display screen 30 should preserve the polarizationstate of the light such that polarization of the return light issubstantially the same as the polarization of the incident light. It iswell known that display surfaces can be constructed using materials andfabrication techniques that substantially preserve the polarization ofthe incident light. The present invention can be applied to a projectionapparatus 10 that projects a polarized light beam by distributing thefluorescent agent 27 over a reflective layer 26 that is substantiallypolarization preserving. In this case, it is important that thefluorescent agent 27 or the protective coating 28 not introduce anysignificant degree of depolarization. Measurements made on the displayscreen samples described earlier show that they are substantiallypolarization preserving.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 projection apparatus-   12 r, 12 g, 12 b spatial light modulator-   14 dichroic combiner-   16 r, 16 g, 16 b light source-   20 projection lens-   22 light beam-   24 fluorescent dot-   25 substrate-   26 reflective layer-   27 fluorescent agent-   28 protective coating-   30 display surface-   40 wavelength band-   42 incident wavelength band-   44 emissive wavelength band-   46 return light wavelength band-   52 measured speckle contrast-   54 mean code value-   62 measured speckle contrast-   64 mean code values-   72 green reflection peak-   82 green reflection peak-   84 green fluorescence band-   92 green reflection peak-   94 green fluorescence band-   102 blue reflection peak-   104 green reflection peak-   106 red reflection peak-   112 blue reflection peak-   114 green reflection peak-   116 red reflection peak-   118 green fluorescence band-   122 blue reflection peak-   124 green reflection peak-   126 red reflection peak-   128 green fluorescence band-   132 z(λ) color matching function-   134 y(λ) color matching function-   136 x(λ) color matching function-   150 viewer-   200 color calibration process flow chart-   202 measure spectral response of primaries step-   204 calculate colorimetry of primaries step-   206 determine primary matrix step-   208 determine normalized primary matrix step-   210 store normalized primary matrix step-   212 determine primary power adjustment factors step-   214 adjust primary power levels step-   O optical axis-   λ1, λ2 peak wavelength-   Δλ1, Δλ2 bandwidth-   Δλs Stokes shift

1. A projection display surface for reducing speckle artifacts from aprojector having at least one narrow band light source having anincident visible wavelength band, wherein the incident visiblewavelength band has an incident peak wavelength and an incidentbandwidth, comprising: a) a substrate having a reflective layer thatreflects incident light over at least the incident visible wavelengthband; and b) a fluorescent agent distributed over the reflective layer,wherein the fluorescent agent absorbs a fraction of the light in theincident visible wavelength band and emits light in an emissive visiblewavelength band having an emissive peak wavelength and an emissivebandwidth; wherein return light from the projection display surfaceproduced when incident light in the incident visible wavelength band isincident on the projection display surface contains light in both theincident visible wavelength band and emissive visible wavelength band,thereby reducing image artifacts, and wherein the projection displaysurface is polarization preserving such that polarization of the returnlight is substantially the same as the polarization of the incidentlight.
 2. The projection display surface of claim 1 wherein the reducedimage artifacts are speckle artifacts or observer metameric failureartifacts.
 3. The projection display surface of claim 1 wherein theemissive bandwidth is wider than the incident bandwidth and is at leastfive nanometers in width.
 4. The projection display surface of claim 1wherein the emissive bandwidth is no more than 50 nanometers.
 5. Theprojection display surface of claim 1 wherein the emissive peakwavelength is shifted relative to the incident peak wavelength.
 6. Theprojection display surface of claim 5 wherein the emissive peakwavelength is shifted by no more than 40 nanometers relative to theincident peak wavelength.
 7. The projection display surface of claim 1wherein the fluorescent agent is a fluorescent dye.
 8. The projectiondisplay surface of claim 7 wherein the fluorescent dye is rhodamine 6G,Alexa Fluor 532 or BODIPY 530/550.
 9. The projection display surface ofclaim 1 wherein the fluorescent agent is a quantum dot.
 10. Theprojection display surface of claim 9 wherein the quantum dots arefabricated using CdS, CdSe, ZnSe, InAs, GaAs or GaN.
 11. The projectiondisplay surface of claim 1 wherein the fluorescent agent is uniformlydistributed on the substrate.
 12. The projection display surface ofclaim 1 wherein the fluorescent agent is sparsely distributed on thesubstrate.
 13. The projection display surface of claim 1 wherein anamount of the fluorescent agent distributed over the reflective layer isadjusted to substantially minimize a speckle visibility.
 14. Theprojection display surface of claim 1 wherein the substrate is made of areflective material, and wherein the reflective layer corresponds to thetop surface of the substrate.
 15. The projection display surface ofclaim 1 further including a protective coating layer that protects theprojection display surface.
 16. The projection display surface of claim15 wherein the fluorescent agent is included in the protective coatinglayer.
 17. The projection display surface of claim 15 wherein theprotective coating layer is applied over the top of the fluorescentagent.
 18. (canceled)
 19. The projection display surface of claim 1wherein light in the incident visible wavelength band and the emissivevisible wavelength band are perceived by a human observer to have thesame color name.
 20. The projection display surface of claim 1 whereinthe fraction of the light in the incident visible wavelength band thatis absorbed by the fluorescent agent is between 2% and 40%.
 21. Theprojection display surface of claim 1 wherein the speckle artifacts arereduced by the mechanism of spectral broadening.
 22. The projectiondisplay surface of claim 1 wherein the emissive peak wavelength islonger than the incident peak wavelength, and further including a secondfluorescent agent distributed over the reflective layer, wherein thesecond fluorescent agent absorbs a fraction of the light in the incidentvisible wavelength band and emits light in a second emissive visiblewavelength band having a second emissive peak wavelength and a secondemissive bandwidth, the second emissive peak wavelength being shorterthan the incident peak wavelength.
 23. The projection display surface ofclaim 1 further including one or more light absorbing agents distributedover the reflective layer which absorb ambient light in one or morewavelength bands which do not substantially overlap with the incidentvisible wavelength band or the emissive visible wavelength band.
 24. Aprojection display surface for reducing speckle artifacts from a colorprojector having at least three narrow band light sources having first,second and third incident visible wavelength bands, respectively,wherein each incident visible wavelength band has an associated incidentpeak wavelength and an associated incident bandwidth, comprising: a) asubstrate having a reflective layer that reflects incident light over atleast the first, second and third incident visible wavelength bands; andb) a first fluorescent agent distributed over the reflective layer,wherein the first fluorescent agent absorbs a fraction of the light inthe first incident visible wavelength band and emits light over a firstemissive visible wavelength band having a first emissive peak wavelengthand a first emissive bandwidth; wherein return light from the projectiondisplay surface produced when incident light in the first incidentvisible wavelength band is incident on the projection display surfacecontains light in both the first incident visible wavelength band andfirst emissive visible wavelength band, thereby reducing speckleartifacts, and wherein the projection display surface is polarizationpreserving such that polarization of the return light is substantiallythe same as the polarization of the incident light.
 25. The projectiondisplay surface of claim 24 wherein the emissive bandwidth is wider thanthe incident bandwidth and is at least five nanometers in width.
 26. Theprojection display surface of claim 24 wherein the first emissive peakwavelength is closer to the first incident peak wavelength than toeither the second or third incident peak wavelengths.
 27. The projectiondisplay surface of claim 24 wherein the first emissive visiblewavelength band extends to no less than 30 nanometers away from thesecond or third incident peak wavelengths.
 28. The projection displaysurface of claim 24 wherein the fraction of the light in the firstincident visible wavelength band that is absorbed by the firstfluorescent agent is between 2% and 40% and the fraction of the light inthe second and third incident visible wavelength bands that is absorbedby the first fluorescent agent is negligible.
 29. The projection displaysurface of claim 24 further including: c) a second fluorescent agentdistributed over the reflective layer, wherein the second fluorescentagent absorbs a fraction of the light in the second incident visiblewavelength band and emits light over a second emissive visiblewavelength band having a second emissive peak wavelength and a secondemissive bandwidth; wherein return light from the projection displaysurface produced when incident light in the second incident visiblewavelength band is incident on the projection display surface containslight in both the second incident visible wavelength band and secondemissive visible wavelength band.
 30. The projection display surface ofclaim 29 further including: d) a third fluorescent agent distributedover the reflective layer, wherein the third fluorescent agent absorbs afraction of the light in the third incident visible wavelength band andemits light over a third emissive visible wavelength band having a thirdemissive peak wavelength and a third emissive bandwidth; wherein returnlight from the projection display surface produced when incident lightin the third incident visible wavelength band is incident on theprojection display surface contains light in both the third incidentvisible wavelength band and third emissive visible wavelength band. 31.(canceled)